Magneto-resistive sensors for determination of angles or positions are known. The document WO 02/06845 A1 provides a summary of the prior art relating to analog determination of angles by means of magneto-resistive sensors on the basis of the anisotropic magneto-resistive (AMR) effect, the gigantic magneto-resistive (GMR) effect and the tunnel effect between magneto-resistive layers (TMR). In the case of the AMR effect, the resistance of a layer strip is governed by the angle a between the current direction and the direction of magnetization. This can be defined by the relationship:R( )=R0+(□R/2)(1−cos(2))  Equation (1)
As can be seen from this, the resistance change passes through one complete period after a change in the angle through 180°. If the direction of magnetization matches the direction of the field, acting on the layer strip, of a permanent magnet which is mounted such that it can rotate in the vicinity, then two complete period cycles of the resistance change are produced for one complete revolution of the permanent magnet. This can be measured as a voltage change when current flows through the layer strip.
In the case of the GMR effect and TMR effect, the resistance of a layer strip or of a tunnel transition is dependent on the angle □ between the directions of the magnetization of two layers or layer components of magneto-resistive material. If the magnetization direction in the one layer component is fixed by means of a natural or artificial anti-ferromagnet which is in direct contact and only the magnetization direction of the second component follows the applied field of the permanent magnet which can rotate, then the resistance change is defined by the relationship:R(□)=R0+(□R/2)(1−cos(□))  Equation (2)
In this case, this results in only one full period of the resistance change for one revolution of the permanent magnet, or of the voltage, which can be used as a signal, on the layer strip or tunnel junction.
In accordance with the stated formulae, this results in ideal conversion of an angle to a cosine function. In practice, however, it has been found that this conversion cannot be carried out without errors. Harmonics of the respective fundamental occur in the signals and their period lengths are an integer fraction of the 180° or 360° mentioned above. The reason for the errors is that the assumed match between the direction of the magnetization of the layers and the direction of the applied magnetic field is not sufficiently accurate. In the case of AMR angle sensors, the match between the stated directions can be improved by high permanent magnetic field strengths. However, this can be achieved only by using expensive high-coercivity permanent magnet materials, or by mechanically complex fitting of the permanent magnet in the very immediate proximity of the sensor. In the case of GMR or TMR sensors, the use of ever higher magnetic strengths does not lead to an improvement in the sinusoid nature of the signal, since these field strengths result in rotation of the magnetization of the magneto-resistive layer which is coupled to the anti-ferromagnet.
There are two possible reasons for the discrepancy in the direction of the magnetization of the free and AMR layer strips from the direction of the applied magnetic field. The first is fundamentally of a physical nature and has already been mentioned in Laid-Open Specification DE 198 39 450 A1 with the equation which the angle (1) between the direction of the magnetization and the strip longitudinal direction must satisfy, and which is given byHx/H0 tan(□)+sin(□)−Hy/H0=0  Equation (3)
Hx is in this case the magnetic field component in the strip longitudinal direction, and Hy is the component at right angles to this. The match between the angle □ and the angle stated above for which the equation:tan( )=Hy/Hx  Equation (4)
applies is achieved only in the situation in which Hx/H0, and Hy/H0 assume very large values, which corresponds to the stated condition of the need to use high field strengths. The second reason is that, in limited angle ranges and with relatively weak fields from the permanent magnet, the magnetization direction is split into domains, particular at strip ends and edges, and this leads to resistances which differ from the ideal behavior and to hysteresis in the angle ranges.
The two cited documents WO 02/06845 A1 and DE 198 39 450 A1 have specified arrangements of layer strips, which each form magneto-resistive resistors, in order to improve the measurement accuracy which can be achieved by means of magneto-resistive angle sensors, and these arrangements are suitable for filtering out the harmonics from the output signals from the sensors. This filtering is also effective at relatively low magnetic field strengths. This is achieved in that, in the case of AMR sensors, two or more groups of parallel straight strips with precisely defined angles between the longitudinal directions of the strips are used instead of long straight magneto-resistive strips whose longitudinal directions are parallel to one another in sensors without harmonic filtering. The greater the number of groups of strips that are used, the higher is the order of harmonics up to which the harmonics are filtered out of the signal. However, one disadvantage is that each new group of strips with a new fixed inclination angle with respect to the already existing strips also results in new angle ranges of the applied magnetic field, whose angle must be measured, and in which splitting into domains and thus hysteresis occurs. This applies in particular to field strengths which are not very much greater than the anisotropic field strength of the strips.
It is admittedly normally possible to use parallel long magneto-resistive strips in the case of GMR and TMR sensors. However, the directions of the magnetization of the layers which are coupled to the respective anti-ferromagnet in the various groups of strips must form precisely defined angles between them. It is thus fundamentally impossible to avoid magnetization components which are aligned at right angles to the strip longitudinal direction, and, in this case as well, this results in the formation of a number of angle ranges, which increases with the number of groups of strips in which hysteresis occurs, particularly in the case of the relatively weak field strengths which can be produced economically.
Magneto-resistive sensors for determination of positions, which are arranged close to the surface of magnetic scales such that they can move in the measurement direction, determine the respective position value from the angle which the magnetic field forms with a strip direction at the location of the sensor. If the magnetic scale is composed of a permanent magnet material which is magnetized periodically in alternate directions, then this angle varies approximately linearly with progress in the measurement direction. The problems which have to be solved in order to achieve high measurement accuracies correspond essentially to those in the case of angle sensors. This applies primarily to use for filtering of harmonics.